1. Field of the Invention
The present invention relates to a method for manufacturing an inductance in a monolithic circuit. More specifically, it relates to the manufacturing of an inductance in a monolithic circuit integrating a limited number of passive elements, such as inductors, resistors and capacitors, and possibly a limited number of active elements, such as a protection diode.
2. Discussion of the Related Art
FIG. 1 shows a cross-section view of a monolithic circuit including an inductance 10 formed conventionally. A semiconductor 11, for example, made of silicon, is covered with a multiple-layer formed of an insulating layer 12, for example, silicon oxide, and of first and second passivation layers 13, 14. Inductance 10 rests on the external surface of the first passivation layer 13.
FIG. 2 shows a cross-section view of inductance 10 along lines A-A of FIG. 1. Inductance 10 is formed of a conductive track 15 arranged in a spiral and having first and second ends 16 and 17, first end 16 being located substantially at the center of the spiral. Second end 17 is connected to a pad 20. First end 16 is connected to a pad 21, via vias 22, 23, formed in first passivation layer 13, and a metal portion 24 deposited on insulating layer 12. Pads 20 and 21 belong to a set of pads intended for the direct assembly of the upper surface of the monolithic circuit on a printed circuit.
Inductance 10 is formed after forming in substrate 11 of possible active components. Inductance 10 may be obtained by the deposition, on first passivation layer 13, of a metal layer, for example, aluminum, covered with a mask, and then anisotropically etched. The portions of the metal area which are not protected by the mask form conductive track 15 and ends 16, 17 of inductance 10. Finally, second passivation layer 14 is deposited and pads 20 and 21 are formed. First passivation layer 13 especially has the function of moving inductance 10 away from substrate 11 to limit the couplings therebetween.
Inductance 10 has a series resistance having the following formula:R=ρ×l/S where ρ is the resistivity of the material forming inductance 10, l is the length of conductive track 15, and S is the surface area of the cross-section of conductive track 15. Conventionally, the cross-section of conductive track 15 is substantially rectangular and S is defined by S=d×w, where d corresponds to the thickness of conductive track 15, that is, substantially the thickness of the metal deposited on first passivation layer 13, and w is the width of conductive track 15 as is shown in FIG. 1. Inductance 10 also exhibits a quality factor which corresponds to the ratio between the exchanged power (magnetic power) and the lost power (ohmic losses). Its value is given, for frequencies smaller than 500 Mhz, by the following formula:Q=L×ω/RDC where ω is the pulse of the current flowing through inductance 10, L is the value of the inductance and RDC is the equivalent series resistance of the low-frequency inductance (that is, for frequencies smaller than 500 MHz), which is equal to the series resistance provided by the above formula.
According to the previously-described conventional method of forming of an inductance 10, the obtained values of the series resistance of inductance 10 exhibit a dispersion of approximately 5%, due to the dispersions cumulated both by the metal deposition step on first passivation layer 13, and the metal etching. This dispersion causes a dispersion of the quality factor of the inductance. Practically, such a dispersion may cause matching problems upon subsequent use of the monolithic circuit.
Further, thickness d of inductance 10 is limited both by the technology used for the metal deposition and by the fact that this deposition must be covered by second passivation layer 14. Indeed, due to the presence of inductance 10, the surface on which second passivation layer 14 is formed exhibits an irregular relief that may cause stress distribution inhomogeneities in second passivation layer 14, that may result in layer breakages. This is why thickness d is generally maintained at less than 3 μm. This makes difficult the obtaining of very small series resistance values and high quality factors, if it is not desired to increase width w and thus the surface area taken up by the inductance.